Generally, a wireless communication system uses one of a vertical polarization signal, a horizontal polarization signal, and a circular polarization signal, or a multiple polarization signal to reduce interference between systems and to increase capacity of the same frequency band in a wireless communication system that provide a wireless communication service such as a ground wireless service and a satellite service.
FIG. 1 is a schematic diagram illustrating a wireless communication system transmitting and receiving a linear polarization signal according to the related art.
Referring to FIG. 1, the wireless communication system transmitting/receiving a linear polarization signal according to the related art includes a satellite 101, a ground terminal 105 for transmitting and receiving vertical or horizontal polarization signals, and a ground antenna 103 for transmitting and receiving polarization signals from the ground terminal 105.
In case of downlink 107, the satellite 101 transmits a linear polarization signal 111 and has limited transmission power not to give interference to the other satellite network or terrestrial network. If the transmission power of the satellite 101 is limited, a signal may be attenuated due to linear polarization distortion 113, thereby deteriorating system performance. For example, the ground terminal 105 capable of receiving a circular polarization signal receives a signal from the satellite with 50% power reduced when the ground terminal 105 receives a distorted linear polarization signal. That is, power reduction caused by linear polarization distortion may deteriorate overall system performance if the transmission power of the satellite 101 is limited.
In case of uplink 109, the ground terminal 105 transmits a linear polarization signal 113 and has limited transmission power not to give interference to the other satellite network or ground network. If the transmission power of the ground terminal is limited like the downlink 107, a signal is attenuated by polarization distortion, thereby deteriorating system performance.
As a technology using a polarization tracking device according to the related art, a technology for increasing a signal to noise ratio through removing cross-polarization components was introduced. A communication system using horizontal and vertical polarization can simultaneously transmit two channels without interference using the same frequency.
However, a polarization tracking apparatus, used in the described technology according to the related art, cannot control polarization that randomly varies due to environmental factors between a satellite and a ground terminal and posture variation according to movement of a ground terminal. Thus, it is difficult to receive a signal with no polarization distortion at a ground terminal. Due to such a reason, a horizontal (or vertical) polarization receiver receives a part of vertical (or horizontal) polarization component and has cross polarization components. Therefore, a signal quality of a desired channel is degraded.
As another technology using a polarization tracking device according to the related art, a technology for tracking an arrival direction of a transmission source using instantaneous of polarization changing in time was introduced. Here, a dual-polarized array element is used for receiving polarization changing in time.
However, such a technology according to the related art cannot control polarization changing randomly by environmental factors because it was introduced under assumptions that polarization linearly changes in time and the arrival direction of a transmission source also linearly changes in time.
Therefore, a transmitter or a receiver of a wireless communication system employs a device for estimating and controlling polarization in order to reduce signal attenuation by correcting polarization distortion in the receiver or the transmitter, which may occur due to environmental factors between a satellite and a ground terminal or posture variation according to the ground terminal.
As another technology according to the related art, two slots are disposed to cross each other in a microstrip antenna, and a diode is connected to the crossing thereof. Then, an angle of polarization is controlled by closing and opening the diode.
In this technology, it is impossible to control linear polarization distorted in a predetermined angle because only orthogonal polarization is possible.
As described above, the polarization tracking device according to the related art cannot control a polarization signal varying randomly in a predetermined direction due to transmission power limitation, environmental factors between communication systems, and posture variation, thereby causing signal attenuation.